OSI LAYER DEFINITIONS

I. DEFINITIONS

The main problem in the communication between computers from different vendors is because they use protocols and data formats are different. To overcome this, the International Organization for Standardization (ISO) to create a communication architecture known as the Open Systems Interconnection (OSI) model defines a standard for connecting computers from different vendors.

The OSI model consists of 7 layers, and the second layer also has a number of sub-layer (divided by the Institute of Electrical and Electronics Engineers (IEEE)). Consider the following table:

 

  

II . LAYER FUNCTIONS1 . Physical layerIt is the simplest layer ; related to electrical ( and optical ) connections between equipment . Binary data encoded in a form that can be transmitted through the network media , for example cables , transceivers and connectors associated with the Physical layer . Equipment such as repeaters , hubs and network cards are located in this layer .

2 . The data - link layerThis layer a little more " intelligent " than the physical layer , as it provides real data transfer . As a liaison between the media and the network layer protocol that is more high - level , data link layer is responsible at the end of the binary data packet from a higher level to a discrete package prior to the physical layer . Will send the frame ( block of data) through a network . Ethernet ( 802.2 and 802.3 ) , Tokenbus ( 802.4 ) and TokenRing ( 802.5 ) is a protocol in the data - link layer .

3 . Network layerThe main task of the network layer is to provide routing functionality so that packets can be sent out from a local network segment to a destination that is on a different network . IP , Internet Protocol , commonly used for this task . Other protocols such as IPX , Internet Packet eXchange . Novell company has been programmed into the protocol , such as SPX ( Sequence Packet Exchange ) and NCP ( NetWare Core Protocol ) . This protocol has been incorporated into the Netware operating system . Some functions may be performed by the Network Layer

    Dividing the binary data stream into discrete packets with a certain length
    detecting Error
    Fix the error by sending reset packets corrupted
    controlling the flow

4 . Transport layerData transport layer , using protocols such as UDP , TCP and / or SPX ( Sequence Packet eXchange , which one is used by NetWare , but specifically for connection -oriented IPX ) . Transport layer is the center of fashion - OSI . This layer provides a reliable and transparent transfer between two end points , this layer also provides multiplexing , flow control and error checking and repair .

5 . Session layerSession layer , as the name suggests , is often mistaken as the network and logon procedures related to safety . This layer provides services to the layer above two , Coordinate communication between the entities layer represents. Some protocols at this layer : NETBIOS : a session interface and protocol , developed by IBM , which provides services to the presentation layer and application layer . NETBEUI , ( NETBIOS Extended User Interface ) , a development of that used on the NETBIOS Microsoft networking products , such as Windows NT and LAN Manager . ADSP ( AppleTalk Data Stream Protocol ) . PAP ( Printer Access Protocol ) , which is contained in the Postscript to access the printer on the AppleTalk network .

6 . Presentation layerPresentation layer of the OSI model perform only a single function : translation of various types in the system syntax . For example , a connection between the PC and the mainframe requires conversion from EBCDIC character- encoding format to ASCII and many factors to consider . Data compression ( and encryption that may be ) handled by this layer .

7 . Application layerThis layer is the most " intelligent " , the gateway is at this layer . Gateway does the same job as a router , but there is a difference between them . Application Layer is the primary interface between the application running on one computer and network resources that require access to it . Application layer is the layer in which the user would operate him , protocols such as FTP , telnet , SMTP , HTTP , POP3 is at the Application layer .